In mufflers and engines used as power sources of portable working machines such as a bush cutter and a blower, a so-called forced air cooling type of performing cooling with cooling air by a cooling fan attached to one end of a drive shaft is widely used. In order to prevent workers from abruptly coming into contact with muffler portions, a muffler cover is often provided to cover almost the whole of the muffler. In this case, rises in the temperatures of the muffler and the muffler cover become a problem. In the related art, in order to avoid the temperature rise problem, a gap between the muffler and the muffler cover are set as large as possible. However, in this case, the size of the muffler cover increases. In order to solve this problem, it is effective to efficiently cool the muffler during operation of the engine, and make it difficult for heat of the muffler to be transferred to the muffler cover. For example, in PTL 1, a heat shield plate is disposed above a muffler, and a portion of cooling air of a cylinder is introduced into the muffler cover. By virtue of the heat shield plate, PTL 1 enables cooling the muffler while suppressing the heat of the muffler from being transferred to the muffler cover.